WEBVTT

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MARKUS KLUTE: Welcome back
to special relativity.

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Bending of light is one of
the spectacular consequences

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of general relativity, a
prediction, if you want,

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[? of ?] [? general ?]
relativity,

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which was experimentally
confirmed.

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But let's analyze
the situation first

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before we look at
the actual evidence.

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They use Alice and
Bob again, where

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Alice is an elevator operator.

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She's stationary and
operating this elevator.

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Bob is a passenger and
riding within the elevator.

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So Alice is injecting
a light beam

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and watching this light beam
in the elevator from outside.

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And the light beam,
as you would expect,

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is going in a straight line.

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We'll see pictures here show
the elevator at three different

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times-- [? t1, ?]
[? t2, ?] and t3.

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For Bob, the very same situation
looks completely different.

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He sees the light
entering the elevator.

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And then after some
time, the light pulse

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is maybe on the
height of his head.

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And after some additional time,
he sees the light at the end--

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or at the bottom
part of the elevator.

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So if you would draw a line
on the back of the elevator

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where he was able to
observe the light pulses,

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you could draw this line.

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So for Bob, who's stationary
in this accelerating reference

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frame, light is bending.

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As we discussed in the first
section in this chapter,

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there's an equivalence
between accelerating

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and the gravitational field
causing an acceleration.

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And so you can use heavy
objects, like the Sun,

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in order to bend light.

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And this led to the
first observation

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of the rotational
bending of light.

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And so in order to
do this, we want

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to have a star very
close, or starlight

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passing very close to the Sun.

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And because the Sun is very
bright, you want to use solar--

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a total solar eclipse in
order to test this effect.

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And this was first
achieved in 1919

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with a solar eclipse
in Brazil, also

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at the West Coast of
Africa, by Arthur Eddington.

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And so this then led to
Einstein really becoming famous.

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I talk about this later.

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You might remember the
last partial eclipse here

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in Massachusetts in 2017.

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I have a vivid memory of
this, showing this to my kids

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up into the sky, obviously,
with proper eye protection.

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So the idea is, again,
that you have a star,

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and light passes by the Sun.

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And because of the gravitational
bending of the Sun,

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there's a bending effect.

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So there seems to be an
offset of the actual start

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position due to the bending.

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And this effect,
again, was discovered--

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or was measured in 1919 by
Arthur Eddington, and let's--

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maybe because of an article
in The New York Times

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to the fame of Albert Einstein.

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So at the time, there
was no science reporters

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for newspapers.

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So a former sports
reporter reported

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on the scientific endeavor.

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And the way he writes about
it is rather interesting.

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You might want to read
the entire article.

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But here's just the headlines.

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And it read like, "Lights All
Askew in the Heavens," "Men

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of Science More or
Less Agog Over Results

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Of Eclipse Observations,"
"Einstein's

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Theory Trumphs," Stars not Where
They Seemed or Were Calculated

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to Be, But Nobody Need to
Worry," "A Book for 12 Wise

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Men," "No more in all the
world could comprehend it,"

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said Einstein, when his
daring publisher accepted it.

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So, again, this populous writing
made Einstein very popular.

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He was then later
invited to come and visit

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to the United States and
made a tour, which he also

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used for political reasons.

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But, really, it made
him very, very famous--

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made him a pop star of the time.